1. Field of the Invention
The present invention relates to configurations of optical fibers that provide the ability to assemble, spool and unspool, deploy or use such configurations, while maintaining the fiber's ability to transmit laser energy over distances, and in particular, over great distance and at high powers. The present invention further relates to configurations that are strengthened to withstand harsh environments, such as the environments found in a borehole, a nuclear plant, or under the sea. In particular, the present invention relates to unique and novel configurations utilizing additional fiber length to minimize bending losses while providing benefits for selected predetermined applications.
As used herein, unless specified otherwise “high power laser energy” means a laser beam having at least about 5 kW (kilowatt) of power. As used herein, unless specified otherwise “great distances” means at least about 500 m (meter). As used herein the term “substantial loss of power,” “substantial power loss” and similar such phrases, mean a loss of power of more than about 3.0 dB/km (decibel/kilometer) for a selected wavelength. As used herein the term “substantial power transmission” means at least about 50% transmittance.
2. Discussion of Related Art
Until the development of the inventions set forth in patent application Ser. No. 12/706,576, pending, filed Feb. 16, 2010, the entire disclosure of which is incorporated herein by reference, it was believed that the transmission of high power laser energy over great distances without substantial loss of power was unobtainable. As a consequence, prior to the inventions of that patent application it was further believed that there was no reason to construct, or investigate the composition of, an optical fiber, an optical fiber configuration, or an optical fiber cable for the transmission of high power laser energy over great distances.
Power loss over long distances occurs in an optical fiber from many sources including: absorption loss, and in particular absorption loss from hydroxyl ions (OH−); Rayleigh scattering; Brillouin scattering; Raman scattering; defects; inclusions; and bending loss. These problems have been documented in the literature.
An example of the prior belief in the art that a paradigm existed between the transmission of high power laser energy over great distances and substantial power loss, is illustrated in the article by Muto et al., titled “Laser cutting for thick concrete by multi-pass technique,” CHINESE OPTICS LETTERS Vol. 5, Supplement May 31, 2007, pages S39-S41 (hereinafter referred to as “Muto”). Although Muto states that 4 kW of power were delivered down a 1 km fiber, when 5 kW of laser power was put into the fiber, Muto fails to eliminate the stimulated Raman scattering (“SRS”) phenomena. As shown by Muto's paper this deleterious phenomenon will effectively clamp the output power as length or power is increased. The SRS phenomenon is shown by the spectrum in FIG. 3 of Muto. Thus, prior to the invention of Ser. No. 12/706,576, it was believed that as input laser power, or the length of the fiber increased, the power output of a fiber would not increase because of the stimulated Brillouin scattering (“SBS”), SRS and other nonlinear phenomena. In particular, SBS would transfer the output power back up the fiber toward the input. Further, SBS, SRS, as well as the other deleterious nonlinear effects, in addition to limiting the amount of power that can be transmitted out of the fiber, can result in fiber heating and ultimate failure. Thus, as recognized by Muto, at page S41 “[i]t is found that 10-kW-power delivery is feasible through a 250-m-long fiber with the core diameter of 150 μm. The physical phenomenon which restricts the transmitted power is SRS.” Thus, Muto, as did others before him, failed to deliver high power laser energy over great distances.
Further, Muto does not disclose, discuss or address the placing of its optical fiber in any protective tubing or material, the coiling and uncoiling of its fiber or the strengthening of its fiber for use in a particular application. In particular, Muto does not address the bending losses associated with such configurations and, in particular, the bending losses that are associated with strengthened configurations.
The present invention provides solutions to bending loss problems that are associated with configuring optical fibers in protective structures and, in particular, in placing long lengths of high power optical fibers in protective tubing and then coiling and uncoiling such a configuration. Various solutions, examples of which are provided in this specification, are provided for minimizing, and in certain instances eliminating to any practical extent, bending losses that result from such configurations.
The present invention advances the art of laser delivery, and in particular the art of high power laser delivery, by providing an optical fiber configuration that avoids or mitigates the bending losses associated with optical fiber configurations and, in particular, provides an optical fiber configuration for the transmission of high power laser energy over great distances in harsh environments without substantial power loss.